774 research outputs found
On the origin of discrepancies between observed and simulated memory of Arctic Sea ice
To investigate the inherent predictability of sea ice and its representation in climate models, we compare the seasonal-to-interannual memory of Arctic sea ice as given by lagged correlations of sea-ice area anomalies in large model ensembles (Max Planck Institute Grand Ensemble and Coupled Model Intercomparison Project phase 6) and multiple observational products. We find that state-of-the-art climate models significantly overestimate the memory of pan-Arctic sea-ice area from the summer months into the following year. This cannot be explained by internal variability. We further show that the observed summer memory can be disentangled regionally into a reemergence of positive correlations in the perennial ice zone and negative correlations in the seasonal ice zone; the latter giving rise to the discrepancy between observations and model simulations. These findings could explain some of the predictability gap between potential and operational forecast skill of Arctic sea-ice area identified in previous studies
Testrun results from prototype fiber detectors for high rate particle tracking
A fiber detector concept has been realized allowing to registrate particles
within less than 100 nsec with a space point precision of about 0.1 mm at low
occupancy. Three full size prototypes have been build by different producers
and tested at a 3 GeV electron beam at DESY. After 3 m of light guides 8-10
photoelectrons were registrated by multichannel photomultipliers providing an
efficiency of more than 99%. Using all available data a resolution of 0.086 mm
was measured.Comment: 18 pages, 17 figure
Flow Phase Diagram for the Helium Superfluids
The flow phase diagram for He II and He-B is established and discussed
based on available experimental data and the theory of Volovik [JETP Letters
{\bf{78}} (2003) 553]. The effective temperature - dependent but scale -
independent Reynolds number , where
and are the mutual friction parameters and the superfluid Reynolds
number characterizing the circulation of the superfluid component in units of
the circulation quantum are used as the dynamic parameters. In particular, the
flow diagram allows identification of experimentally observed turbulent states
I and II in counterflowing He II with the turbulent regimes suggested by
Volovik.Comment: 2 figure
Differential Calculi on Commutative Algebras
A differential calculus on an associative algebra A is an algebraic analogue
of the calculus of differential forms on a smooth manifold. It supplies A with
a structure on which dynamics and field theory can be formulated to some extent
in very much the same way we are used to from the geometrical arena underlying
classical physical theories and models. In previous work, certain differential
calculi on a commutative algebra exhibited relations with lattice structures,
stochastics, and parametrized quantum theories. This motivated the present
systematic investigation of differential calculi on commutative and associative
algebras. Various results about their structure are obtained. In particular, it
is shown that there is a correspondence between first order differential
calculi on such an algebra and commutative and associative products in the
space of 1-forms. An example of such a product is provided by the Ito calculus
of stochastic differentials.
For the case where the algebra A is freely generated by `coordinates' x^i,
i=1,...,n, we study calculi for which the differentials dx^i constitute a basis
of the space of 1-forms (as a left A-module). These may be regarded as
`deformations' of the ordinary differential calculus on R^n. For n < 4 a
classification of all (orbits under the general linear group of) such calculi
with `constant structure functions' is presented. We analyse whether these
calculi are reducible (i.e., a skew tensor product of lower-dimensional
calculi) or whether they are the extension (as defined in this article) of a
one dimension lower calculus. Furthermore, generalizations to arbitrary n are
obtained for all these calculi.Comment: 33 pages, LaTeX. Revision: A remark about a quasilattice and Penrose
tiling was incorrect in the first version of the paper (p. 14
Genuine Counterfactual Communication with a Nanophotonic Processor
In standard communication information is carried by particles or waves.
Counterintuitively, in counterfactual communication particles and information
can travel in opposite directions. The quantum Zeno effect allows Bob to
transmit a message to Alice by encoding information in particles he never
interacts with. The first suggested protocol not only required thousands of
ideal optical components, but also resulted in a so-called "weak trace" of the
particles having travelled from Bob to Alice, calling the scalability and
counterfactuality of previous proposals and experiments into question. Here we
overcome these challenges, implementing a new protocol in a programmable
nanophotonic processor, based on reconfigurable silicon-on-insulator waveguides
that operate at telecom wavelengths. This, together with our telecom
single-photon source and highly-efficient superconducting nanowire
single-photon detectors, provides a versatile and stable platform for a
high-fidelity implementation of genuinely trace-free counterfactual
communication, allowing us to actively tune the number of steps in the Zeno
measurement, and achieve a bit error probability below 1%, with neither
post-selection nor a weak trace. Our demonstration shows how our programmable
nanophotonic processor could be applied to more complex counterfactual tasks
and quantum information protocols.Comment: 6 pages, 4 figure
Algebraic description of spacetime foam
A mathematical formalism for treating spacetime topology as a quantum
observable is provided. We describe spacetime foam entirely in algebraic terms.
To implement the correspondence principle we express the classical spacetime
manifold of general relativity and the commutative coordinates of its events by
means of appropriate limit constructions.Comment: 34 pages, LaTeX2e, the section concerning classical spacetimes in the
limit essentially correcte
- âŠ